Ion-conductive solid electrolyte cells are usually made of a material which provides sufficient ion conductivity only when the material has reached a temperature of 400.degree. C. or higher. Below that temperature, the ion conductivity is usually insufficient for practical evaluation. When used in connection with exhaust gases from internal combustion engines, contamination of the cell due to additives in the gases is an additional problem. For example, lead is frequently contained in the exhaust gases. Solid electrolyte cells have to reach a temperature of 500.degree. C. and more before their function is not impaired by lead contained in exhaust gases. For rapid operation under starting conditions, and generally for reliable operation, it has therefore been proposed to provide the sensors with heating elements.
U.S. Pat. No. 3,546,086 describes a sensor which utilizes a reference gas and which is equipped with a heater element, located in the interior of a tubular solid electrolyte body, spaced from the inner wall thereof. Such a construction is expensive to manufacture and not suited for mass production; it is difficult to so construct such an article that it is immune to malfunction due to vibration or shock, frequently encountered in automotive environments. It has also been proposed--see German Disclosure Document DE-OS No. 23 04 464--to utilize electrochemical sensors having ion conductive solid electrolyte bodies which do not require a reference gas.